There is a layer which meets all the requirements: DLC (Diamond-Like Carbon). With a thickness of < 0.5-0.6 microns, this layer is transparent. However, DLC is applied on the surface by PVD (CVD) .

You can test Polyimide as protective layer

There are very limited amount of organic materials that able to withstand high temperature (>500). As candidate can be Polyimide (may withstand ~500oC but at higher temperature may quickly degradate), poly-para xylylene (but this material has low modulus’ so probably not good candidate for scratch resistance), Poly-dimethyl diphenyl siloxane (may survive up to 450-500oC).

Best regards